This invention relates to a process for the treatment of a waste liquid which contains boron compounds and organic compounds and, more particularly, to a process for recovering boric acid from such a waste liquid by the combustion thereof.
Boron compounds such as metaboric acid and derivatives of boron oxide are widely used as catalysts for a variety of chemical reactions such as partial oxidation of paraffins or cycloparaffins with oxygen. According to such an oxidation process, the oxidation reaction mixture is hydrolyzed with water and the resulting mixture is separated into an organic phase and an aqueous phase. The oxidation end product is obtained from the organic phase while boric acid is recovered from the aqueous phase for reuse as catalyst.
In such processes, a waste liquid still containing boron compounds and organic compounds is produced. In a process for the production of cyclohexanol and cyclohexanone by the liquid phase oxidation of cyclohexane with oxygen in the presence of metaboric acid, for example, there is produced as a waste liquid a solution containing 2.5-10 wt % of boric acid and 15-60 wt % of organic compounds such as adipic acid. Because such waste liquid has a high COD value and is harmful to living beings, the disposal thereof causes environmental pollution.
Combustion is known to be one of the effective methods for the treatment of organic compouds-containing waste liquids. When a waste liquid containing boron compounds as well as organic compounds is treated by conventional combustion methods, however, various problems are found to arise due to the presence of the boron compounds. That is, by the combustion of the waste liquid, the boron compounds are converted into boron oxide which is present in the form of liquid particles in the resulting combustion gas. These particles tend to deposit on the walls of a combustion furnace and combustion gas passages, causing corrosion of the walls and clogging of the passages, whereby the discontinuace of the operation results.